Makefile: introduce BR_PATH

Since the HOST_PATH and TARGET_PATH variables almost contain the same
things, let's factorize this in a single BR_PATH.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>